The present invention relates generally to a method and apparatus for inspecting turbine engines. More specifically, the present invention relates to method and apparatus for inspecting the surface contour of an edge of a cutout formed in a disk for fixedly receiving a turbine blade.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high-pressure turbine are mounted on one shaft, and the fan and low-pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The turbine is composed of a disk having a plurality of complex, closely toleranced slots machined along the periphery of the disk to receive a portion of the turbine blades referred to as a dovetail. The contour of slots is typically formed by broaching. Broaching is a production technique whereby a cutter, typically referred to as a broach, is used to finish cutouts or apertures of circular, square or irregular sections. In broaching, the action of the broach itself serves as a clamping medium so that the operation may be completed in a minimum of time. The cutting tool or broach is typically provided with many teeth graded in size such that each takes a small chip when the tool is directed through the previously prepared leader aperture.
In the case of disks for securing the dovetails of turbine blades, broaching produces an amount of disk material tear out that extends past the surface of the disk opposite the surface in which the broach is first directed, or exit side of the disk. This tear out is in the form of jagged discontinuities or burrs typically formed along substantially the entire periphery of the broached cutout or aperture. Material along the broached area contains reduced strength properties. Since this broached region is highly stressed in operation, care must be taken to remove these discontinuities to avoid the possibility of stress-induced cracking, which can significantly reduce component life. The process of removing the tear out discontinuities is referred to as breaking the edge or producing an edgebreak or breakedge. Material was typically removed from the slot peripheries, such as by machining techniques, to form small rounded edges along the exit side of the disk. Unfortunately, it has been recently discovered that the rounded edges of a considerable number slot peripheries were of insufficient size to remove discontinuous regions formed by broaching, and that follow-up machining or other techniques of material removal is now required to enlarge the radius of the rounded edge or form a chamfer to preserve component life. Thus, a significant number of disks of turbine engines in operation are likely in need of this retrofit operation. To determine whether the disk requires retrofit machining, an inspection is conducted. These disk re-inspections are in addition, of course, to the on-going need to inspect newly fabricated turbine disks.
Further complicating matters, commonly used techniques for inspecting edgebreaks are time-consuming and prone to error. For example, a wax-and-trace technique involves manually placing heated wax to the surface of the disk, waiting for the wax to cool, properly removing the wax, i.e., without deforming or distorting the wax impression, and then visually aligning the impression for mechanical trace to ensure that the trace direction is normal to contour centerlines. Not only did this technique expend up to ten minutes for each peripheral slot, but gage repeatability and reproducibility approached 50 percent. In other words, the impression associated with the wax-and-trace technique, even if done correctly, could account for about one half of the total permissible range of dimensional variation of the edge contour of the slot. Due to the time-consuming nature of the wax-and-trace technique, inspections for edgebreaks may be further limited, thus increasing the probability of missing a defect.
What is needed is a method or apparatus for inspecting turbine disks that can be performed more quickly and having improved gage repeatability and reproducibility.